Until the results of the study were first reported at a December 2008 medical conference, nobody expected Rituxan to have such a dramatic effect in CLL. But the study, in 817 CLL patients at 190 cancer centers in 11 nations, surprised experts, who were underwelmed by earlier reports.

A clinical trial "rarely has such a profound effect on the treatment of a disease," says Peter Hillmen, MD, PhD, of the U.K.'s Leeds Teaching Hospitals NHS Trust. Hillmen's editorial comments accompany the official report of the study results in the Oct. 2 issue of The Lancet.

CLL is the most common kind of leukemia in adults. It strikes five out of 100,000 people each year, mostly the middle-aged and elderly. The disease gets worse relatively slowly, and patients become less and less able to fight off infections.

By looking at biologic markers in CLL cancer cells, doctors can judge whether a patient is at high, intermediate, or low risk. Patients at highest risk -- about 8% of people with CLL -- have cancer cells that have lost a marker called p53.

The p53 marker is an Achilles heel that standard chemotherapy exploits. Rituxan does not work through p53, but because the new treatment must be used in combination with standard chemotherapy, it was not much help to patients with p53 loss.

But all others, including those with a marker of poor prognosis called 11q deletion, tend to respond quite well.

In the study, standard treatment with six courses of chemotherapy resulted in 45% survival after three years. Adding Rituxan to this therapy increased survival to 65%.

This treatment "changes the natural course of chronic lymphocytic leukemia," conclude study researchers Michael Hallek, MD, of the University of Cologne, Germany, and colleagues.

The study enrolled patients ranging in age from 30 to 81, but all were physically fit for their age. Because of the rigors of chemotherapy -- which increase with the addition of the new drug -- results may not be as good for patients who are in poor general health.